Petalometry for the ELT: dealing with the wavefront discontinuities induced by the telescope spider

The presence of a six legged 50cm-wide spider supporting the secondary mirror of the Extremely Large Telescope (ELT) breaks the spatial continuity of the incoming wave-front. Atmospheric turbulence, low wind effect and thermo-mechanical drift of the deformable mirror are all potential contributors to discontinuities between the six segments of the ELT pupil. It is therefore necessary to measure these differential pistons in order to reconstruct the full wave-front. The pyramid wave-front sensor is currently the preferred design for adaptive optics systems. However, it was shown to be a poor differential piston sensor in the visible, under partial turbulence correction, leading to a severe degradation of the image quality. Using the COMPASS adaptive optics (AO) simulator, we first investigate strategies to ensure the spatial continuity of the correction applied on the deformable mirror. These methods present some limitations in strong seeing conditions, when the corrugated phase varies a lot below the spider legs, and lead to a significant degradation of the Strehl Ratio. To tackle this critical issue, we propose as a second step to couple the continuity hypothesis with a petalometer: a sensor specifically designed for sensing the differential piston. As candidates, we compare an unmodulated pyramid, a Zernike wavefront-sensor and a Zernike coupled with a field stop. We present results regarding their sensitivity and their reliability when working in operation, in presence of realistic AO residuals.